We use a range of cosmological data to constrain phenomenological modifications to general relativity on cosmological scales, through modifications to the Poisson and lensing equations. We include cosmic microwave background anisotropies measurements from the Planck satellite, cosmic shear from CFHTLenS and DES-SV, and redshift-space distortions from BOSS data release 12 and the 6dF galaxy survey. We find no evidence of departures from general relativity, with the modified gravity parameters constrained to $\Sigma = -0.01_{-0.04}^{+0.05}$ and $\mu = -0.06 \pm 0.18$. We also forecast the sensitivity of the full five-year Dark Energy Survey and of an LSST-like experiment to those parameters, showing a substantial expected improvement in the constraint on $\Sigma$.